A challenge in the construction of electrodes for electrochemical cells, such as those used in rechargeable batteries, is achieving a good electrical connection between the conductive elements of the electrode and the corresponding current collector. Current collectors are typically made of thin sheets of metal, such as aluminum. As part of forming the electrode, a calendaring process is employed in which the electrode material is pressed into the current collector. The high mechanical forces of the calendaring procedure result in particles of the electrode material becoming mechanically adhered to the current collector, essentially deforming the metal sheet and forming a mechanical interlock between the current collector and the remainder of the electrode.
U.S. patent Ser. No. 11/396,515, entitled “Nanoscale Ion Storage Materials”, which is incorporated by reference in its entirety, discloses nanoscale materials useful for electrochemical cells. These materials have much smaller particle size, and correspondingly higher surface area, than conventional, coarse-grained electrode materials. The high surface to volume ratio or specific surface area, as well as their smaller crystalline dimensions, provide fundamentally different physical properties compared to their coarse-grained counterparts. Electrodes made with nanoscale materials pose new challenges in manufacturing, however. For example, the high-pressure calendaring process is insufficient in itself to achieve mechanical adherence of nanoscale electrode materials to the current collector. The material properties of slurries used in the preparation of composite electrode layers prepared from nanoscale materials are altered due to the smaller particle size. This difference, as well as the higher surface area of nanoscale materials, inhibits the formation of a mechanical interlock between the particles and the current collector during calendering. Thus, because of the altered material properties of nanoscale compositions, as well as the material's unique dimensions (i.e., lacking the macroscopic edges and points of conventional electrode materials), the particles cannot be pressed into the current collector to form the mechanical interlock achieved by larger-scale conventional materials.
Moreover, to achieve the desired rheology for coating the current collector surface, slurries of nanoscale electrode materials are necessarily more dilute (i.e., having a higher solvent fraction) than slurries made from larger-scale electrode materials. As these slurries dry, there is a tendency for the binder to separate from the current collector, resulting in less effective anchoring of the electrode to the collector, further impairing the electrical connection between the collector and the electrode material. In addition, the volume change during drying of such slurries is higher than slurries prepared from conventional larger-scale materials, raising the potential for undesired cracking and spalling of the electrode during drying.
Treatment of the current collector, such as through coatings, has focused on materials that are insoluble in the solvents used in the electrode manufacturing process. Prior to the invention described herein, soluble coatings or adhesives were considered undesirable because solubilization at the current collector-electrode interface was thought to result in the loss of adhesion between these two materials. Moreover, current collector coatings have the potential for reducing the electrical connectivity between the current collector and the electrode active material.
U.S. Pat. No. 5,554,459 provides a coating composition for the current collector of an electrode. The coating composition includes polyolefinic based compositions, such as poly(ethylene-co-acrylic acid) copolymers, all of which are not soluble in common electrode casting solvents (e.g., NMP). The coating composition separates the active material of the electrode from the current collector, potentially resulting in reduced electrical connectivity between the current collector and the active material of the electrode.